Hello, my name is***** be patient whilst awaiting responses, I'm not constantly available.
The building regulations 2010 Combustion Appliances and Fuel Storage Systems Approved Document J2. state that the type and amount of ventilation is as follows... a permanently open air vent with a total equivalent area of at least 50% of the cross-sectional area of the flue (ie. 8" flue then 4" permanently open air vent) must be in place.
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The flue liner fitted to the chimney is 6'' dia and the air inlet pipe under the floor is 3'' dia, this appears to meet the requirement you show, please let me know if the additional 150mm air vent in the wall is not necessary.
According to the regulations, the current air supply pipe to your system seems adequate ... I can only advise you to contact you Local Building Control at you County Council offices to gauge their opinion regarding the air-brick requirement. You can now access the advice of your local Building Control office free of charge.